Sonar scienceTheories abound when it comes to the science of sonar-related whale injury. Some research draws a direct correlation between sonar and marine mammal behavior. Others say sonar is just a fraction of the trouble and that other factors — disease, parasites, algal blooms, pollution, trauma and oceanographic events — are bigger threats.

ShipsideAs of 2004, about 60 percent of the Navy’s surface ships and submarines were equipped with mid-frequency active sonar — defined as frequencies between 1 and 10 kilohertz — as were some helicopters and fixed-wing aircraft. Active sonar sends sound waves through the water that strike what’s in their path and can be analyzed to identify, track and target the obstacle. The military uses this sparingly because it reveals a ship’s position. The more secure option is passive sonar — listening in with hydrophones.

The Navy must test its high-powered active sonar, officials have said, because 40 countries have submarines that may be quiet enough to escape traditional sonar systems.

WhalesMarine mammals listen and transmit sound through water. They depend on sound to find food, avoid predators, navigate, communicate and find mates. According to some scientists, an acoustic disturbance such as sonar interferes with these processes. In other cases, like in the Bahamas in 2000, sonar is thought to cause whales to become disoriented and end up stranded in shallow water. In the Bahamas, seven out of 13 stranded whales died after a naval sonar exercise. This is the only sonar-related stranding for which the Navy claims responsibility.

That incident — and many other stranding events — involved Curvier’s beaked whales. The Navy now avoids cutting off egress routes for whales and avoids known beaked whale concentration areas, according to the Navy’s “Understanding Whales and Sonar” Web site.

Other scientists believe that sonar causes whales to panic and rise too quickly, creating air bubbles in their organ tissue — like in “the bends,” or decompression sickness.

Sources:U.S. Navy Web site (www.whalesandsonar.navy.mil), news reports.

— Allison Batdorff

Sonar factsSonar is an acronym for “Sound Navigation and Ranging.”

¶ Passive sonar is a listening device that uses hydrophones (underwater microphones) that receive, amplify and process underwater sounds.

¶ Active sonar emits pulses of sound that travel through the water, reflect off objects and return as an echo to an underwater acoustic receiver.

¶ Of the U.S. Navy’s approximately 280 surface ships, only about 58 percent are equipped with mid-frequency active sonar. About half of these ships are underway at any given time, and for each ship, active sonar is turned on only a small percentage of the time (during certain types of training and maintenance activities).

Types of active sonar

High-frequency: High-frequency sonar (greater than 10 kilohertz) is primarily used for determining water depth (fathometers), hunting mines, and guiding torpedoes. At higher frequencies, the sound energy is greatly attenuated (weakened due to scattering and absorption) as it travels through the water. This results in shorter ranges, typically less than 5 nautical miles.

Mid-frequency: Mid-frequency sonar, which includes the AN/SQS-53 system, has been in use since World War II, and is the primary tool for identifying and prosecuting submarines. Mid-frequency sonar (between 1 and 10 kilohertz) suffers moderate attenuation and has a typical range of 1-10 nautical miles.

Low-frequency: Low-frequency sonar (less than 1 kilohertz) produces sound that suffers less attenuation as it travels through the water, providing greater range than other sonars. Achieving ranges up to 100 nautical miles, low-frequency sonar is primarily used for long-range search and surveillance of submarines. Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) is the U.S. Navy’s low-frequency sonar system.

Sonar configurations

Sonar systems can be mounted to the hulls of various combat ships, towed behind ships in an array, dipped into the water from helicopters, or attached to free-floating buoys (sonobuoys).

From the U.S. Navy Web site: www.whalesandsonar.navy.mil/